The main difference is that cannot is much more common than can not. The only exception is in the type of expression discussed above - 'You can NOT go if you don't want to go' - here cannot wouldn't work.- i.e. it is possible not to go.

"We can not do it if you don't want to!" "Ani, lets do it." My competitive spirit rised up. "Sure." He smiled cheerfully. "We'll try the competition please." "Okay!"

(on SEO - to someone who had created a lot of keywords) "first of all, you can not make so many keywords, you need to focus on some of them."

"he will get pissed whenever I ask if he can not spend so much time on the computer,"This is quite a good one - constructions like 'Can you not make so much noise' are quite common, but less so without the intervening pronoun - could not is more common (being reported speech).

Incidentally:"can not do it if you don't want" gets only 1 hit on Google"can not go if you don't want" -gets 4, but one of them is this page and two are obviously written by non-native speakers

@Thredder -There is a theoretically possible difference yes, as in 'I can not go, you know. I don't have to.' But trying to find examples in real life is nigh on impossible. The vast majority of times that can not is used it has exactly the same meaning as cannot (see quote from Oxford Dictionaries in my November 12, 2012 comment).

Incidentally, I've just noticed a comment by Mr Eugene from November 11, 2012 which neatly and wittily shows another difference of use:

'I cannot believe I found a blog about the words "cannot" and "can not" , this can not be happening....lol.'

'cannot' for unstressed, and 'can not' when we really want to stress 'not'.

@Abootty - I don't know where you got that one from, as 'cannot' is the most common on both sides of the Atlantic:

OED (UK) - "cannot is the ordinary modern way of writing can not"AskOxford (UK) - "Both cannot and can not are acceptable spellings, but the first is much more usual."

Common Errors, at Washington State University (US) - "These two spellings are largely interchangeable, but by far the more common is “cannot” and you should probably use it except when you want to be emphatic"Merriam-Webster (US) - "cannot"GrammarGirl (US) - 'Both "cannot" and "can not" are acceptable, although it's more common to see the one-word spelling--"cannot." '

@nadibes - The problem for me is that your example "I was going to X, but if you don't want me to I can not" doesn't sound particularly natural to me. Wouldn't we be more likely to say "but if you don't want me to, I can *not* go", or even more likely "I don't *have* to go"?

In fact there is not one single hit on Google (or in Google Books) for "if you don't want me to I can not", whereas there are nearly five million for "if you don't want me to I don't have to". Even "You can not go if you don't want to", which sounds slightly better to me, really only gets one hit.

If we did use "can not" in this sense, it is because we are stressing the "not", and "cannot" would admittedly not be possible here, and there would indeed be a difference in meaning. But I think that sort of sentence is pretty rare.

As you rightly say, this stress also happens with "can you not", which can sometimes be a stronger version of "can't you".

"Can you *not* shut up while I'm trying to listen to the news?""Can't you shut up while I'm trying to listen to the news?"

But in other cases, "can't" wouldn't be possible.

"Can you *not* forget to buy milk this time?"

I've just noticed dbfreak's example - "Can you not shake your leg when I’m in the room?", where he says that it means - You can just not shake, ok? -> You can not shake it. As in, you can choose to not shake it rather than you being unable, incapable of shaking!

But that's not what it means to me at all! To me it means "Please don't shake your leg when I’m in the room!" just as "Can you not forget to buy milk this time?" means "Please don't forget".

A search at the British National Corpus for "can not" brings up a random fifty examples - 48 of which are for "cannot", and where "cannot" or "can't" could be substituted in both examples "can not".

I find the distinction quite useful. Cannot means the action is impossible; can not means there is the option to not take the action. Thus "I was going to X, but if you don't want me to I can not" means I would be happy to change my plans if you want me to"; on the other hand, "I was going to X, but if you do't want me to I cannot" means that you have the power to veto my action.

@e2e4: Actually, the second was in common currency in centuries past--from Jane Austen's Emma: "'Cannot you, my dear Emma--cannot you form a guess as to what you are to hear?'" Cf. will not/do not etc. Austen. It would still be grammatically correct, but modern tongues would automatically contract it: "Can't you go there?" or "Can't you form a guess?"

Indeed, I think that today "Can you not" would be used to mean "Can't you" only by people trying to sound extra-formal, such as authors writing historical fiction (which is often wrong, since it should be "Cannot you" to avoid anachronism). Much more often I would expect a spoken "Can you not?" to have the emphasis on the 'can' (as others have noted), meaning "Are you allowed to not?" or else "Would you refrain?" rather than "Is it impossible?": "Can you not buy homeowner's insurance?" or "Can you not keep tapping your foot?"

@Henri - OK I misunderstood your argument. I thought you meant "may" for permission, but in fact you're talking about possibility. What I think you're really saying is "I might go, then on the other hand I might not go".

But I wouldn't say that that's a very common use of "can not" nor that it's the only possible meaning. We can use it when we want to stress the "not" for example. But Burchfield writing in the New Modern Fowler's suggest that it's also simply a matter of preference; he says he usually writes "can not" rather than "cannot" for example.

@Henri - do you have any evidence to back you up there. Every dictionary I've looked at has cannot =can not, without any qualification.

Merriam-Webster Dictionary of English Usage has several examples of "cannot" meaning "may not", including one from the original language maven, William Safire, who used to write on English at the New York Times - "You can flout convention and flout authority, but you cannot use use flaunt for flout". At the time a correspondent complained to Safire that he should have said "may not", but MWDEU say "this usage is so clear and firmly established in writing on usage that it shouldn't be quibbled at"

There is no suggestion of this difference either in the original Fowle's or in the Third Edition.

Whatever idiot out there is trying to dictate the spelling of 'cannot' obviously needs better things to do with their time. Um, 'can not' looks way better and if people are freaking out about it than they obviously have control issues

"Both the one-word form cannot and the two-word form can not are acceptable, but cannot is more common (in the Oxford English Corpus, three times as common). The two-word form is better only in a construction in which not is part of a set phrase, such as ‘not only ... but (also)’ " Oxford Dictionaries Online

@e2e4 - You bring up an interesting point about the question form, and as you say "cannot" doesn't work there, but for the very good reason that in question form when the auxiliary is not contracted, "not" comes after the inverted subject:

"Didn't she say she was coming?", but "Did she not say she was coming?""Haven't I seen you before?", but "Have I not seen you before?""Won't you do as I ask?", but "Will you not do as I ask?""Can't you go there?", but "Can you not go there?"

But "cannot" is in a class all of of its own. I don't think the fact that we can't/cannot/can not use it in question form has any bearing on which is better in positive statements.

As for your other point, it is fine to use "could" as the past of "can" when talking about general ability in the past - "At the age of four he could already read and write", but not to talk about ability on a a specific occasion - "I locked myself out last night, but was able to get in through a back window" - NOT - "I could get in through a back window". In negatives though, "couldn't" works for both general and specific ability in the past - "I couldn't find my keys last night".

Modals don't really have tense, but on certain occasions, such as reported speech, four modals can act as the past of others, so: will>would, can>could, shall>should, may>might.

Incidentally my grammar books warn against simply seeing "could" and "would" as the past of "can" and "will", as they also have several other functions, like for example, distancing - "Could you pass me the jam, please?"

Can not and cannot have 2 different meaning. You have the ability to not do something, which is where you would use "can not", for example "I can not choose to be a jerk, and I an choose to be nice." The word "cannot" indicates impossibility. I cannot life a building.

While it is possible for "can not" and "cannot" to be used to mean the same thing, (I can not eat this food vs I cannot eat this food), it is MUCH simpler to use "cannot" in terms of impossibility, and "can not" when there is a decision to be made on the issue in question.

They are both valid. They do not have quite the same meaning. Think about it. "Cannot" spoken as a single word is just a negation. But "can not" has more force, it is a negation with emotional force because it mirrors the way humans slow down when speaking in anger.

As for "can not" being a contradiction. This is false reasoning. Logically, "can not" means "not can" which is how the logic is expressed in computer languages (and formal logic). Nothing wrong with it.

It is also contradicting to write "need not". One need do something and then not do something. But if you needn't do something, it simply states that one does not need to do something. Put the words together, or it is a contradiction. Simple as that.

It is a contradicting statement to write "can not" . One can do something and then not do something. But if you cannot do something it simply states that one is not able to do something. Simple as that.

OK Muhydeen, let's correct you.The two example sentences where you wrote "can" and "not" side by side have nothing in common with "cannot": I can not only sing but also dance. I can not only teach mathematics but also economics and English.In these sentences the "not" negated the exclusion expressed by the word "only" rather than the possibility expressed by “can".

Dear dbfreak,"Can you not shake your leg when I’m in the room? You can just not shake, ok? -> You can not shake it."First, you probably debate a permission to shake my leg when you are in the room, which should be expressed with the word "may" rather than the physical possibility expressed by "can". However you are correct to state that your example is also correct grammatically. Only then it means that you are possibly so huge as to make shaking a leg physically impossible by your presence in the tiny room. Alternatively, you could be one scary ... whose presence paralyses the poor victim and he or she can not move the darn leg.

I strongly believe that cannot is different from can not. cannot is the negative form of can. I cannot swim. can not is a correlative conjuction- I can not only sing but also dance. I can not only teach mathematics but also econoics and English. I stand to be corrected. Thanks

Well, my dictionary says both are fine. And, a lot of grammarians on the Web seem to agree. But what I'm curious about is how "cannot" became acceptable and then a preferred form. I would guess that at first "can not" was the only acceptable form. We don't use "maynot", "couldnot" or "shouldnot", so why "cannot"?